The present invention relates to an audio signal amplifier, and more particularly to a complementary single-ended push-pull amplifier.
As an amplifier for obtaining a low distortion characteristic, a Class A amplifier and a Class AB amplifier are known. FIG. 1 shows a basic circuit of the Class A push-pull amplifier. In the circuit, a bias voltage V.sub.E /2+V.sub.E /2 is applied between the bases of complementary transistors Q1 and Q2 from bias voltage sources 1 and 2 for operating the transistors Q1 and Q2 in the Class A mode. In the circuit, collector currents I1 and I2 vary in response to a voltage Vi between a connection point of the voltage sources 1 and 2 or an input terminal and the common emitter of the transistors Q1 and Q2, as shown in FIG. 2A. With zero signal input, the collector currents of the transistors Q1 and Q2 are I.sub.D1 and I.sub.D2, respectively. The difference between the currents I1 and I2 is supplied to a load Rl such as a loudspeaker, as an output current Io. Since, in such an amplifier, a crossover distortion is not produced over the entire operating range, a distortion factor, especially at small output power levels, is remarkably reduced. The smaller the output power, the larger the collector power dissipation (the collector current in zero signal input condition is selected one half of a maximum output current and the dissipation in this condition is at maximum). Therefore, the power efficiency of the Class A amplifier is remarkably poor, and requires power transistors of high maximum power dissipation rating and large heat sinks.
The Class AB amplifier circuit is so arranged as to improve the above-mentioned disadvantages of the Class A amplifier, in which a base bias voltage V.sub.E /2+V.sub.E /2 is selected for Class AB operation, as shown in FIG. 2B. In the Class AB amplifier, its efficiency can be improved with a distortion factor at small output power levels kept approximate to that in the Class A operation. When an input signal level becomes larger, however, since either of the transistors Q1 and Q2 is rendered cut-off, the mutual conductance gm of the push-pull circuit becomes about half as large as that when both the transistors are on, thus resulting in increase of especially a third harmonic distortion. As mentioned above, in the prior art amplifier circuits, there is a contradiction that improvement of the distortion factor brings about deterioration of the power efficiency and vice versa.